The following patents are believed to represent the current state of the art:
6,248,112;6,236,875;6,235,038;6,226,548;6,211,904;6,203,497;6,202,646;6,196,225;6,190,395;6,190,382;6,189,533;6,174,281;6,173,199;6,167,145;6,164,277;6,161,537;6,152,909;6,146,402;6,142,144;6,135,948;6,132,372;6,129,683;6,096,050;6,096,050;6,090,040;6,083,213;6,079,731;6,079,409;6,053,166;5,993,424;5,976,072;5,971,997;5,957,844;5,951,571;5,951,461;5,885,248;5,720,275;5,704,987;5,592,939;5,584,795;5,506,912;5,445,161;5,400,771;5,347,987;5,331,967;5,307,804;5,257,636;5,235,970;5,203,320;5,188,111;5,184,603;5,172,225;5,109,830;5,018,509;4,910,590;4,672,960;4,651,746
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